An optical pickup device 50 reads data stored in an optical disc 59 by focusing and inducing a beam from a light source 51 onto the optical disc 59 via a beam splitter 53, a collimating lens 55, and an objective lens 57 and by receiving a reflected light from the optical disc 59 using an optical detector 54, as shown in FIG. 1.
At this time, a linearly polarized light beam is generated from the light source 51 and is then directed to the optical disc 59. The linearly polarized light beam should be converted into a circularly polarized light beam (or an elliptically polarized light beam), and be then irradiated to the optical disc 59. The circularly polarized light beam (or the elliptically polarized light beam) reflected by the optical disc 59 should be converted back to the linearly polarized light beam and then delivered to the optical detector 54 so that data are read.
A phase retardation plate A is disposed to perform the conversion between the polarization states of the beam. The phase retardation plate performs the conversion between the polarization states of the beam by retarding the phase of the transmitted beam on a beam path between the light source 51 and the optical disc 59.
To perform the conversion between the polarization states of the beam, the conventional phase retardation plate A adjusts the amount of phase retardation of the transmitted beam by properly machining the thickness of an inorganic single crystal, such as quartz.
However, with the phase retardation plate A using such an inorganic single crystal, it is very difficult to obtain a desired amount of phase retardation since a phase difference of the transmitted beam significantly depends on an incidence angle. Further, the dependency of the phase difference of the transmitted beam on the incidence angle requires to dispose the phase retardation plate A in a parallel light forming region, resulting in increase in the area of the phase retardation plate A.
Further, it is required to finely adjust the thickness of the single crystal so as to obtain a desired amount of phase retardation. The adjustment of the thickness involves a complicated manufacturing process such as an optical polishing process for the single crystal. Accordingly, there is a problem in that the phase retardation plate is difficult to manufacture and is expensive.
Further, the phase retardation plate A using the conventional inorganic single crystal cannot cope with fluctuation of an oscillation wavelength in the beam due to increase in temperature as the optical pickup device is run over a long period of time. Accordingly, there is a problem in that a desired amount of beam phase retardation is not obtained.
A phase retardation plate for solving such problems is disclosed in Korean Patent Laid-Open Publication No. 2001-0089321. This phase retardation plate has a structure in which a thin organic film with birefringence is bonded, with an adhesive, to a surface of a fixed substrate having the function of transmitting or reflecting light, and a liner thermal expansion coefficient (E1) of the thin organic film, a liner thermal expansion coefficient (E2) of the adhesive and a liner thermal expansion coefficient (E3) of the fixed substrate satisfy the relationships of E1<E2 and E3<E2.
This phase retardation plate uses the thin organic film having a birefringence property for the phase retardation of the beam so that a desired amount of phase retardation is easily obtained without needing a separate machining process. Further, miniaturization is realized because the phase retardation plate has less dependency of the phase difference on the incidence angle. Thus, the phase retardation plate is easily manufactured and production costs thereof are reduced.
Further, since the phase retardation value of the thin organic film can cope with a change in temperature, a desired amount of phase retardation can be obtained even though the oscillation wavelength of the beam is fluctuated due to increase in temperature as the optical pickup device is run over a long period of time.
Although expansion deformation of the thin organic film may be caused by the increase in temperature, the expansion of the thin organic film and the fixed substrate is absorbed by the adhesive since the liner thermal expansion coefficients among the thin organic film, the adhesive and the fixed substrate satisfy the relationships of E1<E2 and E3<E2, thereby preventing the deformation due to the increase in temperature.
However, in such a conventional phase retardation plate using the thin organic film, the liner thermal expansion coefficient (E2) of the adhesive is greater than the liner thermal expansion coefficient (E1) of the thin organic film and the liner thermal expansion coefficient (E3) of the fixed substrate. Accordingly, there is a problem in that the adhesive is significantly changed in length due to the increase in temperature, thereby deteriorating the adhesion reliability of the adhesive.
For example, as disclosed in the aforementioned Patent Laid-Open Publication, the coefficient difference between the adhesive and the fixed substrate becomes about 110 if an adhesive with a liner thermal expansion coefficient (E2) of 1.2×10−4/° C. is used, polycarbonate with a liner thermal expansion coefficient (E1) of 6.2×10−6/° C. is used as the thin organic film, and a glass substrate with a liner thermal expansion coefficient (E3) of 95×10−7/° C. is used as the fixed substrate such that the liner thermal expansion coefficients of the thin organic film, the adhesive and the fixed substrate satisfy the relationships of E1<E2 and E3<E2.
Then, a change in relative length between the adhesive and the fixed substrate due to the change in temperature is found to be about 110 μm when temperature is changed by 1° C. This large length difference degrades an adhesion force, which is dependent on the change in the adhesive length due to frequent changes in temperature. Further, it degrades fixation of the adhesive after the long period of time, thereby causing a peeling phenomenon.
Meanwhile, the conventional phase retardation plate using the thin organic film adjusts polarization states of incident light using a predetermined phase retardation value according to a wavelength band of the incident light. Accordingly, as for a wide wavelength band of incident light, a plurality of phase retardation plates are required to retard the phase of the incident light in a desired wavelength band.